Essential tremor (ET) is the most common movement disorder, affecting an estimated 10 million patients in the U.S., with growing numbers due to the aging population. The prevalence increases with age, increasing from 6.3% of the population over 65, to above 20% over 95. ET is characterized by oscillatory movement, for example between 4-12 Hz, affecting distal limbs, especially the hands. Unlike Parkinson's tremor, which exists at rest, essential tremor is postural and kinetic, meaning tremor is induced by holding a limb against gravity or during movement respectively.
Disability with ET is common, and varies from embarrassment to the inability to live independently as key tasks such as writing and self-feeding are not possible due to the uncontrolled movement. Despite the high disability and prevalence of ET, there are insufficient treatment options to address tremor. Drugs used to treat tremor (e.g., Propranolol and Primidone) have been found to be ineffective in 40% of patients and only reduces tremor by 50%. These drugs also have side effects that can be severe. The alternative treatment is surgical implantation of a deep brain stimulator, which can be effective in reducing tremor amplitude by 90%, but is a highly invasive surgical procedure that carries significant risks and cannot be tolerated by many ET patients. There is thus a great need for alternative treatments for ET patients.
Tremor is also a significant problem for patients with orthostatic tremor, multiple sclerosis and Parkinson's disease. The underlying etiology of tremor in these conditions differs from ET, however treatment options for these conditions are also limited and alternative treatment is warranted.
A number of conditions, such as tremors, can be treated through some form of transcutaneous peripheral nerve stimulation. People have a wide variation in wrist diameters, nerve locations, nerve depolarization characteristics, and skin conduction that leads to challenges in designing a device to comfortably, safely and reliably stimulate the peripheral nerves across a broad population of potential users. For instance, in a wrist-worn device targeting the median, ulnar, and radial nerves at the wrist, the amount of power needed for a given stimulation session can vary widely based on skin impedance and usage scenarios.